1. Field of the Invention
The present invention relates to a composition for electromagnetic wave suppression and heat radiation and to a method for manufacturing the same. More specifically, the present invention relates to a composition for electromagnetic wave suppression and heat radiation for not only suppressing an electromagnetic wave emitted within, for example, an electronic appliance but efficiently achieving heat radiation and to a method for manufacturing the same.
2. Description of the Related Art
Though electronic appliances provided with diverse applications have been developed so far, in recent years, the size of such an electronic appliance tends to become smaller. However, even when the electronic appliance becomes small in size, it is required to cope with the diversity of applications, and therefore, the electric energy (calorific value) used in the electronic appliance does not change so much. For that reason, in recent years, a heat radiation measure within an electronic appliance is much more regarded as important.
As the heat radiation measure (heat measure) within an electronic appliance, heat radiation members such as a heat radiation plate, a heat pipe or a heat sink, each made of a metallic material with a high coefficient of thermal conductivity, for example, copper, aluminum, etc., are widely utilized. Such a heat radiation member with excellent thermal conductivity is in general disposed in a heat generating part (high-temperature area) within an electronic appliance, or disposed within an electronic appliance so as to link a heat generating part (high-temperature area) and a low-temperature area to each other. On that occasion, the heat radiation member and the heat generating part or the like are connected to each other via a highly thermally conductive filler (for example, a heat radiation sheet, etc.). The heat radiation or temperature relaxation within an electronic appliance has hitherto been contrived in such a way.
However, since such a heat radiation member is formed of a metallic material, there is caused, as a side effect, a problem that a harmonic component of an electric signal within the electronic appliance propagates through the heat radiation member, and as a result, an unnecessary electromagnetic wave is radiated from the heat radiation member. Its situation is shown in FIG. 9.
As shown in FIG. 9, when a heat radiation sheet (thermally conductive sheet) 3 is provided between a heat generating part 1 and a metal-made heat radiation member 4 such as a heat sink, heat 6 generated in the heat generating part 1 is transferred into the heat radiation member 4 via the heat radiation sheet and radiated. However, the heat generating part (high-temperature area) 1 within the electronic appliance is mainly configured of a semiconductor device (semiconductor package) with a high current density and the like. That is, the heat generating part 1 with a high current density is an area where an electric field intensity or a magnetic field intensity which may possibly cause heat radiation of an unnecessary electromagnetic wave is large. Therefore, when the heat radiation member 4 is disposed in the vicinity of the heat generating part 1, a magnetic field 2 emitted from the heat generating part 1 and the heat radiation member 4 are coupled with each other, whereby an electromotive force is generated in the heat radiation member 4, and a harmonic noise component 5 is generated. As a result, not only the heat 6 but the harmonic noise component 5 of an electric signal propagates into the heat radiation member 4. That is, the heat radiation member 4 becomes a transfer route of the harmonic noise component 5. On that occasion, since the heat radiation member 4 is made of a metal, there is generated a phenomenon in which the heat radiation member 4 acts as an antenna, thereby radiating the propagated harmonic noise component 5 as an electromagnetic wave toward the outside.
In order to reduce such a heat radiation phenomenon of an unnecessary electromagnetic wave, there has hitherto been proposed a technique for breaking coupling of the magnetic field between the heat generating part 1 and the heat radiation member 4 using an electromagnetic wave suppressing sheet or an electromagnetic wave suppressing sheet having heat radiation properties in place of the heat radiation sheet (see, for example, JP-A-2006-196747, JP-A-2003-209010, JP-A-2003-332113 and JP-A-2002-371138).
In JP-A-2006-196747, JP-A-2003-209010 and JP-A-2003-332113, an electromagnetic wave suppressing sheet is proposed. Such an electromagnetic wave suppressing sheet is configured of a sheet matrix such as a resin or a rubber and a magnetic particle with a high magnetic permeability, which is filled in the inside thereof, such as ferrite. In the electromagnetic wave suppressing sheet, for example, a magnetic field generated from the heat generating part is absorbed by this magnetic particle, thereby suppressing the radiation of an unnecessary electromagnetic wave.
Also, in JP-A-2002-371138, an electromagnetic wave suppressing sheet having heat radiation properties (hereinafter referred to as “electromagnetic wave suppressing and heat radiating sheet”) is proposed. In general, the electromagnetic wave suppressing and heat radiating sheet is prepared by mixing a powder for heat radiation with a high coefficient of thermal conductivity, such as alumina or aluminum nitride and a magnetic powder with a high magnetic permeability, such as ferrite, with, for example, a silicone based or acrylic polymer material (for example, resins or rubbers). That is, in the magnetic wave suppressing and heat radiating sheet, by filling a powder for heat radiation (heat radiating filler) and a magnetic powder (magnetic filler) in a matrix, a heat radiation action and an electromagnetic wave suppression action (decoupling action of the magnetic field) are made compatible with each other by a single sheet.
FIG. 10 shows the situation of propagation of heat and a harmonic noise signal in the case where the foregoing electromagnetic wave suppressing and heat radiating sheet is provided between the heat generating part 1 and the heat radiation member 4. In that case, the magnetic field 2 generated from the heat generating part 1 is absorbed by an electromagnetic wave suppressing and heat radiating sheet 7, and the harmonic noise component 5 generated in the heat radiation member 4 is reduced, too.